Feat: allow overriding base URL for Anthropic credentials

[idirouhab]

Summary

This PR updates the AnthropicApi credential type to support overriding the default base URL (https://api.anthropic.com). This change mirrors the existing flexibility of the OpenAiApi credential, allowing users to specify a custom endpoint.

This is particularly useful for testing against mock environments or using enterprise proxies.

How to test

1. Go to Credentials > Anthropic in n8n.
2. Enter a custom Base URL (e.g., pointing to a mock server).
3. Use the “Test” button to verify that the test request is sent to the provided endpoint.

No visual changes in the UI other than the added field.

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Related Linear tickets, Github issues, and Community forum posts

N/A – improvement identified internally for consistency with existing OpenAI credential functionality.

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Review / Merge checklist

• PR title and summary are descriptive. ([conventions](../blob/master/.github/pull_request_title_conventions.md))
• [Docs updated](https://github.com/n8n-io/n8n-docs) or follow-up ticket created.
• Tests included.
• PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)

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Let me know if there's a Linear ticket you want to include or if you want help writing a test for this.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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